The invention relates to active web stabilization apparatus.
Sheet type products are often manufactured in the form of a continuous web running at high speed between various processing components which may dry, coat or otherwise treat the web. Web support is provided by these components in conjunction with auxiliary rolls. The spaces between support points are known as draws and may, necessarily, be large (many yards). Unsupported in these draws, webs may flutter, billow or otherwise move about with respect to their mean line of travel. Such spurious motion can result in breaks and may otherwise interfere with proper operation of the overall process, particularly in the case of very light weight webs such as tissue and plastic film. The invention relates to non-contact stabilization of such web movement.
U.S. Pat. Nos. 3,587,177 and 3,629,952 describe a drying nozzle and web dryer which provide a means of controlling the travel of a web, without contact, while concurrently drying it. Drying is accomplished by using heated air, but control of the web is achieved by introducing a series of jets as wide as the web, blowing parallel to its path and adjacent to a flat rigid surface.
The action of these jets is to suck the web close to and hold it in proximity to this flat rigid surface. The parallel jets are contrived by means of slot nozzles aimed at an angle to the web, but turned to flow parallel by utilizing the "Coanda effect" along a curved surface. The flow mechanisms at work in this design are elucidated in "Airfoil Web Dryer Performance Characteristics", by Hagen et al., Proceedings of the 1984 TAPPI Coating Conference.
The basic principle that a parallel jet, created using the Coanda effect, can interact with a web has been utilized in a number of web management and related purposes. An example of its use as a web stabilizer is U.S. Pat. No. 3,650,043, and as a web conveyor, U.S. Pat. No. 3,705,676. Web cleaning illustrations are provided in U.S. Pat. Nos. 5,466,298 and 5,577,294. Web threading applications are shown in U.S. Pat. Nos. 3,999,696, 4,147,287, 4,186,860 and 4,726,502.
Web threading applications are only concerned with narrow portions of the web as in a threading tail, and non-contact of the web with the device is not a necessary objective. Web cleaning focuses on high velocity jets for debris removal followed by features which collect and convey the blown air and the entrained dust away from the web for suitable disposal. Local web stabilization, such as for floppy edges, can be achieved with passive devices using the venturi effect working with boundary layer air transported by the web itself as described in U.S. Pat. No. 5,022,166. These devices generally extend in from the web edges for less than a foot.
An attractive means of stabilization applicable to the whole width of the web and acting from one side only to facilitate easy retraction is the passive device of copending U.S. pat. app. Ser. No. 08/685,086. In this case, the stabilizer works on the boundary layer of air carried with the web. It utilizes the streamlined features of an airfoil shape to create the desired parallel stream of air between the stabilizer surface and the web and to divert excess boundary layer air away from the web. It also incorporates special trailing end features to gradually disrupt the suction effect allowing an orderly detachment of the web as it leaves the stabilizer. This device is compact and simple and it is practical to use many of them to stabilize a long web draw.
Since boundary layer thickness is a function of distance travelled from a prior obstruction, the passive stabilizer may be weakly effective when following close to another machine component. These stabilizers can normally accommodate a wrap, without rubbing, only with very low tension webs. Limited web contact is acceptable in some applications. However, as the contact pressure and/or its duration increase, this type of stabilizer may experience wear and may alter the surface characteristics of the web. On tissue machines, such contact may also contribute to dust generation.
Webs, particularly of paper, are not perfectly homogeneous in terms of formation or moisture content. They will frequently have machine direction ripples or local regions where portions of the sheet run somewhat out of the mean plane of motion. This is particularly likely for light weight papers in long draws where the sheet has no cross-machine restraint. When a subsequent roll is encountered, these out of plane regions can gather into permanent wrinkles, thereby impairing the quality of the final product. Bowed rolls are often used to spread the sheet and thereby promote flatness but non-contact devices to accomplish this are needed.
Accordingly, the objective of the invention is to provide a powered non-contact web stabilization apparatus positioned on one side of the web, arranged across the machine up to the entire width of the web, capable of accommodating substantial angles of wrap, locatable without loss of effectiveness immediately following another machine component, applicable in multiple units closely spaced in the machine-direction, and providing a non-contact web spreading function.